A common problem with spiral inducers used within centrifugal pumps and similar devices is that the fluid in the tank in which the centrifugal pump is installed will begin to rotate in the same direction as, and along with, the inducer blades. When this occurs, the fluid does not move up through the inducer as efficiently. This phenomenon can also result in a change in pressure near the inlet of the inducer and increase the amount of net positive suction head (NPSH) required to make the pump continue to work efficiently or properly.
Net positive suction head required (NPSHR) is a measure of the amount of head or pressure required to prevent the fluid from cavitating, i.e., the formation of vapor bubbles in a flowing fluid. It is desirable to prevent cavitation in devices like inducers, impellers and pumps because the fluid vapor bubbles created by cavitation can generate shock waves when they collapse that are strong enough to damage moving parts around them. While a higher NPSHR is desirable to prevent cavitation in an inducer, impeller and pump, a high NPSHR can also generate cavitation in the tank as the fluid level drops. Hence, a low NPSHR is desirable to enable more fluid to be pumped out of the tank or structure. Accordingly, other solutions are required to reduce cavitation at the inlet of an inducer while not increasing the NPSHR.
Inducers are frequently used in cryogenic systems, including storage tanks, rocket fuel pump feed systems, and other similar uses. Inducers are used in such systems to prevent the fluid being moved from cavitating in the impeller or pump, which can occur when there is not enough pressure to keep the liquid from vaporizing. Non-cavitating inducers are used to pressurize the flow of the fluid sufficient to enable the devices to which the inducer is attached to operate efficiently. An excellent discussion of the fluid dynamic properties of inducers is provided by B. Lakshminarayana, Fluid Dynamics of Inducers—A Review, Transactions of the ASME Journal of Fluids Engineering, December 1982, Vol. 104, Pages 411-427, which is incorporated herein by reference.
The techniques used to improve pump performance relative to the operation of inducers vary significantly. For example, Nguyen Duc et al., U.S. Pat. No. 6,220,816, issued Apr. 24, 2001, describes a device for transferring fluid between two different stages of a centrifugal pump through use of a stator assembly that slows down fluid leaving one impeller before entering a second impeller. A different technique is used in Morrison et al., U.S. Pat. No. 6,116,338, issued Sep. 12, 2000, which discloses a design for an inducer that is used to push highly viscous fluids into a centrifugal pump. In Morrison et al., an attempt is made to resolve the problem of fluids rotating with the inducer blades by creating a very tight clearance between the blades of the auger of the inducer and the inducer housing, and configuring the auger blades in such a way as to increase pressure as fluid moves through the device to the pump.
While grooves have been used in inducer designs in the past, they have not been used to help efficiently move the fluid through the inducer. For example, in Knopfel et al., U.S. Pat. No. 4,019,829, issued Apr. 26, 1977, an inducer is illustrated that has a circumferential groove around a hub at the front of the inducer. This design causes turbulence to develop within the grooves of the inducer hub rather than in the fluid outside of the grooves, thereby reducing the tendency of the fluid to pulsate and generate noise.
Grooves are also illustrated and described in Okamura et al., An Improvement of Performance-Curve Instability in a Mixed-Flow Pump by J-Groves, Proceedings of 2001 ASME Fluids Engineering Division, Summer meeting (FEDSM '01), May 29-Jun. 1, 2001, New Orleans, La. In Okamura et al., a series of annular grooves are formed on the inner casing wall of a mixed-flow water pump to suppress inlet flow swirl and therefore passively control the stability performance of the pump. In particular, the J-grooves of Okamura et al. reduce the onset of back flow vortex cavitation and rotating cavitation that can be induced by the flow swirl at the inlet of the inducer.
Okamura et al. acknowledge, however, that increasing the specific speed of mixed-flow pumps has a tendency to make their performance curves unstable and to cause a big hump at low capacities, thus it is stated that it is doubtful that the illustrated technique would be effective for higher specific-speed (i.e., higher flow rate) pumps.